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| United States Patent |
5,235,804
|
|
Colket, III
, et al.
|
August 17, 1993
|
Method and system for combusting hydrocarbon fuels with low pollutant
emissions by controllably extracting heat from the catalytic oxidation
stage
Abstract
A method of combusting a hydrocarbon fuel includes mixing the fuel with a
first air stream to form a fuel/air mixture having an equivalence ratio of
greater than 1 and partially oxidizing the fuel by contacting it with an
oxidation catalyst to generate a heat of reaction and a partial oxidation
product stream. The partial oxidation product stream is mixed with a
second air stream and completely combusted in a main combustor at a
condition at which appreciable quantities of thermal NO.sub.x are not
formed to generate an effluent gas stream, thereby generating an effluent
gas stream containing decreased amounts of thermal and prompt NO.sub.x. A
system for combusting a hydrocarbon fuel includes, in combination, means
for mixing the fuel with a first air stream, a catalytic oxidation stage
containing an oxidation catalyst, means for mixing the partial oxidation
product stream with a second air stream, and a main combustor capable of
completely combusting the partial oxidation product stream.
| Inventors:
|
Colket, III; Meredith B. (Simsbury, CT);
Kesten; Arthur S. (West Hartford, CT);
Sangiovanni; Joseph J. (West Suffield, CT);
Zabielski; Martin F. (Manchester, CT);
Pandy; Dennis R. (South Windsor, CT);
Seery; Daniel J. (Glastonbury, CT)
|
| Assignee:
|
United Technologies Corporation (Hartford, CT)
|
| Appl. No.:
|
701426 |
| Filed:
|
May 15, 1991 |
| Current U.S. Class: |
60/780; 60/723; 60/732 |
| Intern'l Class: |
F02G 003/00 |
| Field of Search: |
60/723,39.822,732,737,746,39.02,39.06
431/7,170,328
|
References Cited
U.S. Patent Documents
| 2655786 | Oct., 1953 | Carr | 60/39.
|
| 2947600 | Aug., 1960 | Clayton.
| |
| 3433218 | Mar., 1969 | Von Wiesenthal.
| |
| 3705492 | Dec., 1972 | Vickers | 60/760.
|
| 3797231 | Mar., 1974 | McLean.
| |
| 3846979 | Nov., 1974 | Pfefferle.
| |
| 3897225 | Jul., 1975 | Henkel et al.
| |
| 3928961 | Dec., 1975 | Pfefferle.
| |
| 3940923 | Mar., 1976 | Pfefferle.
| |
| 3966391 | Jun., 1976 | Hindin et al.
| |
| 3975900 | Aug., 1976 | Pfefferle | 60/723.
|
| 4040252 | Aug., 1977 | Mosier et al.
| |
| 4047877 | Sep., 1977 | Flanagan.
| |
| 4054407 | Oct., 1977 | Carrubba et al. | 431/10.
|
| 4118171 | Oct., 1978 | Flanagan et al.
| |
| 4154567 | May., 1979 | Dahmen.
| |
| 4179880 | Dec., 1979 | Schirmer.
| |
| 4202168 | May., 1980 | Acheson et al.
| |
| 4285193 | Aug., 1981 | Shaw et al.
| |
| 4459126 | Jul., 1984 | Krill et al.
| |
| 4534165 | Aug., 1985 | Davis, Jr. et al.
| |
| 4731989 | Mar., 1988 | Furuya et al.
| |
| 4787208 | Nov., 1988 | DeCorso.
| |
| 4864811 | Sep., 1989 | Pfefferle | 60/39.
|
| 4875850 | Oct., 1989 | Cagnon et al.
| |
| 4983364 | Jan., 1991 | Buck et al.
| |
| 4988287 | Jan., 1991 | Stegelman et al.
| |
| Foreign Patent Documents |
| 0351094 | Jan., 1990 | EP.
| |
| 2103008 | Aug., 1972 | DE.
| |
| 2618961 | Nov., 1976 | DE.
| |
| 1460312 | Jan., 1977 | GB.
| |
Other References
International Search Report for PCT/US92/03771, mailed 02 Oct. 1992; for
the PCT application that corresponds to United States Application
07/701,426 (the present application).
Lefebvre, Athur H., Gas Turbine Combustion, McGraw-Hill, New York, 1983.
pp. 44-57, 481-485.
|
Primary Examiner: Bertsch; Richard A.
Assistant Examiner: Thorpe; Timothy S.
Attorney, Agent or Firm: Romanik; George J.
Claims
What is claimed is:
1. A method of combusting hydrocarbon fuel, comprising:
(a) mixing the fuel with a first air stream to form a fuel/air mixture
having an equivalence ratio greater than 1;
(b) partially oxidizing the fuel by contacting the fuel/air mixture with an
oxidation catalyst in a catalytic oxidation stage, thereby generating a
heat of reaction and a partial oxidation product stream comprising
hydrogen and carbon oxides;
(c) controllably extracting up to about 50% of the heat of reaction from
the catalytic oxidation stage at the same time the fuel is partially
oxidized to control the temperature and composition of the partial
oxidation product stream, wherein the temperature of the partial
composition product stream affects the amount of thermal NO.sub.x formed
in a main combustor downstream of the catalytic oxidation stage, the
composition of the partial oxidation product stream determines the amount
of prompt NO.sub.x formed in the main combustor, and the temperature and
composition of the partial oxidation product stream affect the stability
of a flame in the main combustor;
(d) mixing the partial oxidation product stream with a second air stream;
and
(e) completely combusting the partial oxidation product stream in the main
combustor at a condition at which appreciable quantities of thermal
NO.sub.x are not formed, thereby generating an effluent gas stream,
wherein the temperature and composition of the partial oxidation product
stream are selected to control simultaneously the amounts of thermal
NO.sub.x and prompt NO.sub.x formed in the main combustor and the
stability of the flame in the main combustor, thereby controlling the
total amount of NO.sub.x in the effluent gas stream.
2. The method of claim 1 wherein at least about 3% of the heat of reaction
is extracted in step (c).
3. The method of claim 1, further comprising, transferring the heat
extracted in step (c) to the effluent gas stream, thereby heating the
effluent gas stream.
4. The method of claim 3, further comprising, expanding the heated effluent
gas stream across a turbine, thereby producing power.
5. The method of claim 3 wherein the main combustor comprises a primary
zone and a secondary zone and the heat extracted in step (c) is
transferred to the effluent gas stream in the secondary zone.
6. The method of claim 1 where in the equivalence ratio in the catalytic
oxidation stage is at least about 2.
7. The method of claim 1 wherein the oxidation catalyst is selected from
the group consisting of platinum, rhodium, iridium, ruthenium, palladium,
and mixtures thereof; chromium oxides; cobalt oxides; and alumina.
8. The method of claim 1 wherein the partial oxidation product stream and
second air stream are mixed prior to combustion.
9. The method of claim 1 wherein the partial oxidation product stream and
second air stream are mixed in a diffusion flame.
10. A method of combusting a hydrocarbon fuel in a gas turbine engine,
comprising:
(a) compressing an air stream in a compressor;
(b) controllably dividing the air stream into a first air stream, a primary
air stream, and a secondary air stream;
(c) mixing the fuel with the first air stream to form a fuel/air mixture
having an equivalence ratio greater than 1;
(d) partially oxidizing the fuel by contacting the fuel/air mixture with an
oxidation catalyst in a catalytic oxidation stage, thereby generating a
heat of reaction and a partial oxidation product stream comprising
hydrogen and carbon oxides;
(e) controllably transferring up to about 50% of the heat of reaction from
the catalytic oxidation stage to the secondary air stream to control the
temperature and composition of the partial oxidation product stream,
wherein the temperature of the partial oxidation product stream affects
the amount of thermal NO.sub.x formed in a main combustor downstream of
the catalytic oxidation stage, the composition of the partial oxidation
product stream determines the amount of prompt NO.sub.x formed in the main
combustor, and the temperature and composition of the partial oxidation
product stream affect the stability of a flame in a primary zone of the
main combustor;
(f) mixing the partial oxidation product stream with the primary air
stream;
(g) combusting the partial oxidation product stream in the primary zone of
the main combustor at a condition at which appreciable quantities of
thermal NO.sub.x are not formed, thereby generating a combustion product
stream;
(h) mixing the combustion product stream with the heated secondary air
stream, thereby generating an effluent gas stream; and p1 (i) expanding
the effluent gas stream across a turbine, thereby producing power,
wherein the temperature and composition of the partial oxidation product
stream are selected to control simultaneously the amounts of thermal
NO.sub.x and prompt NO.sub.x formed in the main combustor and the
stability of the flame in the primary zone of the main combustor, thereby
controlling the total amount of NO.sub.x in the effluent gas stream.
11. A system for combusting a hydrocarbon fuel, comprising in combination:
(a) means for mixing the fuel with a first air stream to form a fuel/air
mixture having an equivalence ratio greater than 1;
(b) a catalytic oxidation stage containing an oxidation catalyst capable of
partially oxidizing the fuel to generate a heat of reaction and a partial
oxidation product stream comprising hydrogen and carbon oxides;
(c) means for controllably extracting a portion of the heat of reaction
from the catalytic oxidation stage at the same time the fuel is partially
oxidized to control the temperature and composition of the partial
oxidation product stream, wherein the temperature of the partial oxidation
product stream affects the amount of thermal NO.sub.x formed in a main
combustor downstream of the catalytic oxidation stage, the composition of
the partial oxidation product stream determines the amount of prompt
NO.sub.x formed in the main combustor, and the temperature and composition
of the partial oxidation product stream affect the stability of a flame in
the main combustor;
(d) means for mixing the partial oxidation product stream with a second air
stream; and
(e) a main combustor capable of completely combusting the partial oxidation
product stream to generate an effluent gas stream.
12. The system of claim 11, further comprising, means for transferring the
extracted heat to the effluent gas stream to heat the effluent gas stream.
13. The system of claim 12, further comprising, means for expanding the
heated effluent gas stream across a turbine to produce power.
14. The system of claim 12 wherein the main combustor comprises a primary
zone and a secondary zone and means for transferring the extracted heat to
the effluent gas stream in the secondary zone.
15. The system of claim 11 wherein the oxidation catalyst is selected from
the group consisting of platinum, rhodium, iridium, ruthenium, palladium,
and mixtures thereof; chromium oxides; cobalt oxides; and alumina.
16. The system of claim 11 wherein the means for mixing the partial
oxidation product stream with a second air stream permit the mixing to
occur prior to combustion.
17. The system of claim 11 wherein the means for mixing the partial
oxidation product stream with a second air stream permit the mixing to
occur in a diffusion flame.
18. A system for combusting a hydrocarbon fuel, comprising in combination:
(a) means for controllably dividing a compressed air stream into a first
air stream, a primary air stream, and a secondary air stream;
(b) means for mixing the fuel with the first air stream to form a fuel/air
mixture having an equivalence ratio greater than 1;
(c) a catalytic oxidation stage containing an oxidation catalyst capable of
partially oxidizing the fuel to generate a heat of reaction and a partial
oxidation product stream comprising hydrogen and carbon oxides;
(d) means for controllably transferring a portion of the heat of reaction
from the catalytic oxidation stage to the secondary air stream to control
the temperature and composition of the partial oxidation product stream,
wherein the temperature of the partial oxidation product stream affects
the amount of thermal NO.sub.x formed in a main combustor downstream of
the catalytic oxidation stage, the composition of the partial oxidation
product stream determines the amount of prompt NO.sub.x formed in the main
combustor, and the temperature and composition of the partial oxidation
product stream affect the stability of a flame in a primary zone of the
main combustor;
(e) means for mixing the partial oxidation product stream with the primary
air stream;
(f) a primary zone of a main combustor capable of combusting the partial
oxidation product stream to generate a combustion product stream;
(g) means for mixing the combustion product stream with the heated
secondary air stream;
(h) a secondary zone of the main combustor capable of diluting the
combustion product stream to generate an effluent gas stream.
Description
TECHNICAL FIELD
The present invention relates to a method and system for combusting
hydrocarbon fuels with low pollutant emissions, particularly low NO.sub.x
emissions.
BACKGROUND ART
It has long been known that exhaust gases produced by combusting
hydrocarbon fuels can contribute to atmospheric pollution. Exhaust gases
typically contain pollutants such as nitric oxide (NO) and nitrogen
dioxide (NO.sub.2), which are frequently grouped together as NO.sub.x,
unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates,
primarily carbon soot. Nitrogen oxides are of particular concern because
of their role in forming ground level smog and acid rain and in depleting
stratospheric ozone. NO.sub.x may be formed by several mechanisms. The
high temperature reaction of atmospheric oxygen with atmospheric nitrogen,
particularly at adiabatic flame temperatures above about 2800.degree. F.,
forms NO.sub.x through the thermal or the Zeldovich mechanism ("thermal
NO.sub.x "). The reaction of atmospheric nitrogen with hydrocarbon fuel
fragments (CH.sub.i), particularly under fuel-rich conditions, forms
NO.sub.x through the prompt mechanism ("prompt NO.sub.x "). The reaction
of nitrogen released from a nitrogen-containing fuel with atmospheric
oxygen, particularly under fuel-lean conditions, forms NO.sub.x through
the fuel-bound mechanism ("fuel-bound NO.sub.x "). In typical combustors,
atmospheric oxygen and nitrogen are readily available in the combustion
air which is mixed with the fuel.
While acknowledging a need to control atmospheric pollution, the more
advanced combustion control schemes developed during the past decade were
designed to maximize combustion efficiency to maintain economic operation
with only a secondary regard for pollutant emissions. For example, the
production of CO and UHC was considered undesirable, more because it
indicated poor combustion efficiency than because CO and UHC are
pollutants. To maximize combustion efficiency and flame stability, fuel is
often burned in a diffusion flame at fuel/air ratios as near as possible
to stoichiometric, that is, at equivalence ratios of slightly less than
1.0. The equivalence ratio is the ratio of the actual fuel/air ratio to
the stoichiometric fuel/air ratio. An equivalence ratio of greater than
1.0 indicates fuel-rich conditions, while an equivalence ratio of less
than 1.0 indicates fuel-lean conditions. Burning a fuel at an equivalence
ratio slightly less than 1.0 produces nearly complete combustion,
minimizing CO and UHC production, and a hot flame, maximizing useable
energy. The temperatures produced during such an operation are high enough
to produce appreciable quantities of thermal NO.sub.x. Therefore, the goal
of achieving good thermal efficiency, which arises from economic concerns,
is seemingly at odds with the goal of minimizing NO.sub.x emissions, which
arises from environmental concerns and is required by increasingly
stringent environmental regulations.
Several fairly simple methods are available to decrease NO.sub.x emissions,
although none are entirely satisfactory. For example, the formation of
fuel-bound NO.sub.x can be minimized or avoided entirely by burninq a low
nitrogen or nitrogen-free fuel. However, burning a low nitrogen fuel does
nothing to reduce the formation of thermal or prompt NO.sub.x. The
formation of thermal NO.sub.x can be reduced by operating under uniformly
fuel-lean conditions, such as by using a lean diffusion flame or a lean
premixed/prevaporized (LPP) system. The excess air used to achieve
fuel-lean combustion acts as a diluent to lower flame temperatures,
thereby reducing the amount of thermal NO.sub.x formed. The formation of
prompt NO.sub.x can also be reduced by operating under fuel-lean
conditions because the excess air decreases the concentration of CH.sub.i
available to react with atmospheric nitrogen. However, the extent to which
thermal and prompt NO.sub.x formation can be reduced by fuel-lean
combustion may be limited by flame instability which occurs at very lean
conditions.
Accordingly, what is needed in the art is a method and system for
efficiently combusting hydrocarbon fuels while minimizing pollutant
emissions, particularly NO.sub.x emissions.
DISCLOSURE OF THE INVENTION
The present invention is directed to a method and system for efficiently
combusting hydrocarbon fuels while minimizing pollutant emissions,
particularly NO.sub.x emissions.
One aspect of the invention includes a method of combusting a hydrocarbon
fuel. The fuel is mixed with a first air stream to form a fuel/air mixture
having an equivalence ratio greater than 1 and partially oxidized by
contacting the fuel/air mixture with an oxidation catalyst in a catalytic
oxidation stage, thereby generating a heat of reaction and a partial
oxidation product stream comprising hydrogen and carbon oxides. The
partial oxidation product stream is mixed with a second air stream and
completely combusted in a main combustor at a condition at which
appreciable quantities of thermal NO.sub.x are not formed, thereby
generating an effluent gas stream containing decreased amounts of thermal
and prompt NO.sub.x.
Another aspect of the invention includes a system for combusting a
hydrocarbon fuel which includes, in combination, means for mixing the fuel
with a first air stream to form a fuel/air mixture having an equivalence
ratio greater than 1, a catalytic oxidation stage containing an oxidation
catalyst capable of partially oxidizing the fuel to generate a heat of
reaction and a partial oxidation product stream comprising hydrogen and
carbon oxides, means for mixing the partial oxidation product stream with
a second air stream, and a main combustor capable of completely combusting
the partial oxidation product stream to generate an effluent gas stream.
The foregoing and other features and advantages of the present invention
will become more apparent from the following description and accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a basic combustion system of the
present invention.
FIG. 2 is a schematic representation of a combustion system of the present
invention used in conjunction with a gas turbine engine.
FIG. 3 depicts adiabatic flame temperature and NO.sub.x emissions from the
combustion system depicted in FIG. 2 and a prior art combustion system as
a function of the equivalence ratio in the main combustor primary zone.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention uses a combination of three approaches, partial
oxidation by catalytic means, thermal management, and flame stability
enhancement to control NO.sub.x and other pollutant emissions while
permitting thermally efficient combustion of hydrocarbon fuels in a wide
variety of combustion devices including residential heating units,
industrial process heaters, industrial gas turbines, aircraft gas
turbines, and advanced aircraft engines such as those contemplated for the
high speed civil transport and national aerospace plane projects. These
aspects of the present invention may be better understood by referring to
FIG. 1, which is a schematic of a basic embodiment of the present
invention.
As shown in FIG. 1, an air stream 2, which may be any oxygen containing
stream and may be at any suitable temperature and pressure, may be split
into two smaller streams, a first air stream 4 and a second air stream 6.
The first air stream 4 is mixed with a fuel stream 8, which may be at any
suitable temperature and pressure, to form a fuel/air mixture 10 which has
an equivalence ratio greater than one. The equivalence ratio may be
greater than about 2 and, preferably, will be between about 2.5 and about
8. Most preferably, the equivalence ratio will be between about 3 and
about 5. The fuel may comprise C.sub.1 to C.sub.20 hydrocarbons, C.sub.1
to C.sub.20 hydrocarbon oxygenates, and blends thereof. Suitable gaseous
fuels include natural gas and propane. Suitable liquid fuels include
kerosine, No. 1 heating oil, No. 2 heating oil, and conventional aviation
turbine fuels such as Jet A, Jet B, JP-4, JP-5, JP-7, and JP-8. If the
fuel is a liquid, it should be vaporized or atomized before mixing with
the air or while being mixed with the air. Any conventional means known in
the art may be used to vaporize or atomize the fuel.
The fuel/air mixture 10 flows into a catalytic oxidation stage where it is
contacted with an oxidation catalyst 11 and partially oxidized to generate
a heat of reaction and a partial oxidation product stream 12 comprising
H.sub.2, carbon oxides, primarily CO, and unreacted hydrocarbon fuel.
Catalytic oxidation in this context means a flameless, rapid oxidation or
oxidative pyrolysis reaction carried out at a temperature below that
required to support thermal combustion, that is, conventional combustion
with a flame, and below which thermal NO.sub.x forms in appreciable
amounts. Partial oxidation means that there is insufficient oxygen
available to completely convert the fuel to CO.sub.2 and H.sub.2 O and to
fully liberate the chemical energy stored in the fuel. Partially oxidizing
the fuel to H.sub.2, CO, and other carbon oxides by catalytic means
reduces the amount of hydrocarbon fuel available to form CH.sub.i
fragments in a downstream thermal combustor flame front and therefore,
reduces the amount of prompt NO.sub.x formed in downstream combustion. The
amount of H.sub.2, CO, and unreacted hydrocarbon fuel actually formed
depends on the temperature in the catalytic oxidation stage, which may
range from about 300.degree. F. to about 1800.degree. F. At higher
temperatures, relatively more fuel is converted to H.sub.2 and CO than at
lower temperatures due to changes in the equilibrium product composition.
The oxidation catalyst may be any catalyst capable of partially oxidizing
the fuel. Suitable catalysts include platinum family metals such as
platinum, rhodium, iridium, ruthenium, palladium, and mixtures thereof;
chromium oxides; cobalt oxides; and alumina. Preferably, the catalyst will
be capable of initiating the partial oxidation reaction at the conditions
prevailing in the catalytic oxidation stage, that is, without the addition
of heat from an external source. In some applications, however, the
catalyst may be preheated using a secondary working fluid, resistive
heating, or temporary thermal combustion upstream of the catalyst. The
catalyst may be supported on alumina or a similar substrate and may be in
any conventional form, including granules, extrudates, or a coating on a
metal heat exchanger surface, metal foil, metal honeycomb, or ceramic
honeycomb. The preferred catalysts include platinum family metals,
especially platinum-rhodium deposited on an alumina support. If desired,
more than one catalyst may be incorporated into a graded catalyst bed. The
catalytic oxidation stage may be designed according to conventional
catalytic reactor design techniques.
At the same time the partial oxidation reaction takes place, a portion of
the heat of reaction may be extracted from the catalytic oxidation stage
by heat transfer stream 14 to control the temperature and composition of
the partial oxidation product stream 12. This operation will be referred
to as thermal management. Extracting a small amount of heat or no heat
from the catalytic oxidation stage produces a relatively hot partial
oxidation product stream having a relatively large amount of H.sub.2 and
relatively lower amounts of unreacted hydrocarbon fuel. As a result,
relatively little prompt NO.sub.x will be formed in downstream combustion.
Extracting a relatively large amount of heat from the catalytic oxidation
stage produces a relatively cool partial oxidation product stream having a
relatively lower amount of H.sub.2 and a relatively larger amount of
unreacted hydrocarbon fuel. Any increase in prompt NO.sub.x resulting
from the larger amount of unreacted hydrocarbon fuel will be at least
partially offset by a reduction in thermal NO.sub.x formed in a downstream
thermal combustor where the cooler partial oxidation product stream
produces a lower adiabatic flame temperature. By using thermal management
to control the temperature and composition of the partial oxidation
product stream, the total amount of NO.sub.x formed in the combustion
system can be controlled to suit specific operating conditions. Thermal
management may be used to extract up to about 50% of the heat of reaction
generated in the catalytic oxidation stage. Preferably, up to about 20% of
the heat of reaction will be extracted and, most preferably, about 3% to
about 20% of the heat of reaction will be extracted. If desired, the heat
extraction may take place downstream of the catalytic oxidation stage, in
which case only the temperature of the partial oxidation product stream 12
may be controlled. A heat exchanger may be used to extract a portion of
the heat of reaction. The heat exchanger may be designed according to
conventional heat exchanger design techniques and may be an integral part
of the catalytic oxidation stage or may be a separate unit. The heat
transfer stream 14 may initially be at any temperature which permits heat
to be extracted from the catalytic oxidation stage or partial oxidation
product stream, while its temperature after thermal management will depend
on the amount of heat extracted. The heat transfer stream 14 may be air,
water, or another medium and, after thermal management, can be used in any
capacity for which a person skilled in the art would consider such a
heated stream to be useful. Effective use of the heat transfer stream 14
permits the thermal efficiency of the present invention to be at least as
good as a conventional combustion system.
After catalytic oxidation and thermal management, the cooled partial
oxidation product stream 12 is mixed with the second air stream 6 in a
main combustor and is completely combusted by a thermal combustion
reaction, generating an exhaust gas stream 16. The cooled partial
oxidation product stream may be mixed with the second air stream prior to
combustion or in a diffusion flame. Preferably, the adiabatic flame
temperature in the main combustor will be less than about 2800.degree. F.
to minimize the formation of thermal NO.sub.x. The adiabatic flame
temperature and flame stability characteristics in the main combustor
depend on the temperature and composition of the partial oxidation product
stream and the equivalence ratio in the combustor. In general, the H.sub.2
in the partial oxidation product stream enhances flame stability because
H.sub.2 is lighter and more reactive than the original fuel and mixes
better with the second air stream. Flame stability is especially enhanced
when little or no heat is extracted from the catalytic oxidation stage
because the partial oxidation product stream will contain more H.sub.2 and
will be hotter, leading to better mixing. A more stable flame permits the
main combustor to be operated at a lower equivalence ratio, which produces
a lower adiabatic flame temperature and less thermal NO.sub.x. In any
case, the main combustor should be operated at an overall equivalence
ratio of less than 1.0 to ensure complete combustion. The main combustor
may be any combustor suitable for combusting the partial oxidation product
stream, including a conventional or advanced combustor, and may have
either a single combustion zone or a plurality of combustion zones.
Preferably, the main combustor will be a lean premixed prevaporized
combustor. The main combustor may be designed according to conventional
techniques.
As shown in FIG. 2, combining the present invention with a gas turbine
provides some additional benefits. Air stream 22 enters a compressor and
is compressed to a suitable temperature and pressure. The air exiting the
compressor is controllably divided into three streams, a first air stream
24, a primary air stream 26, and a secondary air stream 28. The first air
stream 24 mixes with a fuel stream 30 to form a fuel/air mixture 32 having
an equivalence ratio greater than 1.0. The fuel/air mixture 32 enters a
catalytic oxidation stage where it is contacted with an oxidation catalyst
33 and partially oxidized to produce a heat of reaction and a partial
oxidation product stream 34 comprising H.sub.2 and carbon oxides. A
portion of the heat of reaction is removed in a heat exchanger by the
secondary air stream 28, heating the secondary air stream 28 and cooling
the partial oxidation product stream 34. The cooled partial oxidation
product stream 34 mixes with the primary air stream 26 and is thermally
combusted in a primary zone of a main combustor at a temperature at which
appreciable quantities of thermal NO.sub.x are not formed to generate a
combustion product stream 36. The fuel/air equivalence ratio in the
primary zone may be greater than 1.0, or less than 1.0, but preferably,
will be less than 1.0 to minimize both thermal and prompt NO.sub.x
formation. The combustion product stream 36 is diluted with the secondary
air stream 28 which may be added to the secondary zone through dilution
holes in the main combustor to generate an exhaust gas stream 38. The
secondary air dilutes and cools the combustion product stream 36 and
returns the heat extracted from the catalytic combustion stage to the
exhaust gas stream 38. As a result, the temperature of the exhaust gas
stream 38, the thermal efficiency of the combustion system, and the
amounts of CO and UHC in the exhaust gas are nearly identical to what they
would have been if a conventional combustion scheme had been used.
Alternately, instead of using all of the secondary air stream 28 as
dilution air, a portion of the secondary air stream 28 may be added to the
primary zone to provide additional combustion air. After exiting the
secondary zone, the exhaust gas stream 38 is expanded across a turbine to
produce shaft work to drive the compressor. The exhaust gases may also be
used for propulsion or to produce additional shaft work.
A system such as that depicted in FIG. 2 can provide gas turbines with
significant additional operating flexibility, particularly when the
turbines are operated off peak power. The improved flame stability
provided by burning a lighter, more reactive fuel in the main combustor
provides wider flammability limits than are available from other fuels,
permitting combustion to be maintained at lower equivalence ratios.
Additionally, the ability to control the division of the air stream into a
primary stream and a secondary stream can be used to provide dynamic
control of the equivalence ratio in the primary zone so that it is kept
constant as power levels are changed.
EXAMPLE 1
A gas turbine engine incorporating a catalytic oxidation stage and a two
zone main combustor as shown in FIG. 2 was modelled on a computer using
conventional techniques which are well known in the art. The catalytic
oxidation stage was represented by a detailed chemical kinetic model, the
main combustor primary zone was represented by a perfectly stirred
reactor, and the main combustor secondary zone was represented by a plug
flow reactor. Compressed air at 18.9 atmospheres and 847.degree. F. was
split into three streams: 7.5% of the air to the first air stream, 42.5%
of the air to the primary air stream, and 50% of the air to the secondary
air stream. The first air stream was mixed with methane, which was at
80.degree. F., to form a fuel/air mixture which had an equivalence ratio
of 4.0 and a temperature of 564.degree. F. The fuel/air mixture was
partially oxidized in the catalytic oxidation stage to produce a partial
oxidation product stream comprising 12 volume percent (vol %) CH.sub.4, 8
vol % CO, and 19 vol % H.sub.2. The residence time in the catalytic
oxidation stage was 20 milliseconds (msec) and the temperature was
maintained at 1340.degree. F. by using thermal management to heat the
secondary air stream to 1192.degree. F. The partial oxidation product
stream, which exited the catalytic stage at 1340.degree. F., was mixed
with the primary air stream in the main combustor primary zone and
thermally combusted with a residence time of 0.1 msec and an equivalence
ratio of 0.6. The combustion product stream, which was at 2750.degree. F.
and contained 6 parts per million (ppm) NO and 6,000 ppm CO, was mixed
with the secondary air stream in the main combustor secondary zone with a
residence time of 6.0 msec and an equivalence ratio of 0.3 to produce an
exhaust gas stream. The exhaust gas stream exited the secondary zone at
2049.degree. F. and contained 3 ppm NO and 6 ppm CO.
EXAMPLE 2
The model from Example 1 was used to model a range of operations in the
main combustor. Conditions in the catalytic oxidation stage were
maintained at 1250.degree. F. and an equivalence ratio of 4 for all cases.
Methane and natural gas were used as the fuels for this example. The
equivalence ratio in the primary zone was varied from 0.6 to 1.5 and the
adiabatic flame temperature was permitted to vary accordingly. The
equivalence ratio in the secondary zone was fixed at 0.3. A model of a
prior art combustion system with an identical main combustor but lacking a
catalytic oxidation stage was also prepared. The model of the prior art
system was run at the same conditions as the first model, except that
methane, instead of a partial oxidation product stream, was fed to the
primary zone. Data from both models are presented in FIG. 3. Curves 31 and
32 and the right hand scale show the primary zone adiabatic flame
temperatures computed for the various primary zone equivalence ratios.
Curves 33 and 34 and the left hand scale show the computed NO.sub.x
concentrations in the secondary zone exhaust gas for the various primary
zone equivalence ratios. Curves 31 and 33 represent operations with the
prior art combustion system. Curves 32 and 34 represent operations with
the present invention. FIG. 3 demonstrates that the present invention can
reduce NO.sub.x emission levels by a factor of three to five at a given
equivalence ratio and can reduce adiabatic flame temperatures by several
hundred degrees at a given equivalence ratio.
The present invention is capable of providing several benefits over the
prior art. First, it provides three techniques, partial oxidation by
catalytic means, thermal management, and flame stabilization, by which
NO.sub.x and other pollutant emissions can be reduced while maintaining
good thermal efficiency. The extent to which any of the three techniques
is used can be varied to optimize the combustion system operation and
design.
Second, because many of the hydrocarbon molecules in the fuel are converted
to H.sub.2 and carbon oxides in the catalytic oxidation stage, fewer
hydrocarbon molecules are available for soot production in the main
combustor. Lower soot production results in fewer particulate emissions
and less radiative heat transfer to combustor walls. Third, the decrease
in adiabatic flame temperatures in the main combustor due to thermal
management, combined with less radiative heating, can prolong the life of
combustor materials or permit the use of less expensive materials.
Fourth, the ability to control the amount of air directed to the primary
and secondary zones of the main combustor permits dynamic control of the
equivalence ratio in the primary zone for off peak operations. Such a
control scheme would be particularly beneficial in gas turbines.
Fifth, the present invention has the flexibility to be used with
rich-burn-quench-lean-burn, or other advanced combustion techniques to
further reduce NO.sub.x emissions.
It should be understood that the invention is not limited to the particular
embodiments shown and described herein, but that various changes and
modifications may be made without departing from the spirit or scope of
the claimed invention.
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